Monday, June 28, 2010

Strawberries Are Way Cool

Father’s Day we did a family hike up Lambs Canyon. I’ve always liked this trail; it’s some of the closest, most easily-accessible, cool, shady, moist, North-slope Wasatch forest to the Salt Lake Valley. AW was in power-hike mode, but I was still groggy from the High Uintas Classic road race the day before, and a monster pancake breakfast the following morning. Soon she and Twin A left Twin B and me far behind, bringing up the rear.

Tangent: Bird Whisperer, at the ripe old age of 11, has proven to be an Early Adopter of the teenaged habits of staying up late, and effecting maximum laziness during the day to compensate. He accomplished this hike by repeatedly striding ahead on his long legs till he caught AW and Twin A, then lying down in the trail and “napping” till Twin B and I caught up.

So I ambled along up the trail with Twin B, who invariably brings up the rear on hikes not because she is slow or lazy, but because she is a chronic dawdler. But dawdling has an upside: she notices more bugs and plants than the other kids. The lower reaches of Lamb’s Canyon trail are the shadiest, with lots of leafy green ground cover trailside. Twin B asked me the names of flowers as we picked our way along, nearly all of which were familiar*.

*Both Canada (pic left) and Blue Violets (pic right)are in bloom in several sports along Lambs Canyon trail right now. Look at me- 2 pics in a footnote!

But about the 4th or 5th flower, I hesitated. It was small, white, 5-petaled and low to the ground. I wasn’t sure I’d seen it in bloom before, but I recognized it from the guides. It was a Woodland Strawberry, Fragaria vesca.

Strawberries are cool for so many reasons. On the one hand, they’re a non-descript little forb sprouting fairly standard little white flowers. But in their form, genetics and history- both natural and cultivated- they’re fascinating.

Woodland Strawberry is one of 2 wild Strawberries growing in Utah, the other being- get ready for it- Wild Strawberry, F, virginiana. Both have broad ranges, and are divided into geographic subspecies. (Our local subspecies of Woodland Strawberry is bracteata.)The ranges of the 2 species are so broad that it’s not quite clear exactly what was their native, pre-settlement range, but it seems pretty certain that Woodland Strawberry is native to both Eurasia and Western North America, while Wild Strawberry is native only to North America.

Side Note: Confusingly, F. vesca in Europe is sometimes referred to as “Wild Strawberry”. But it’s F. vesca they’re talking about. In this post Woodland Strawberry is F. vesca, the diploid species native to both Europe and Western North America, while Wild Strawberry is F. virginiana, the octoploid species native to North America (only).

You can tell Woodland and Wild Strawberries apart by their leaves. Both have toothed leaves, but the central, pointy tooth at the tip of the leaf is much smaller than the other teeth on a Wild Strawberry. On a Woodland Strawberry it’s more or less the same size.

There’s another difference that’s not visible- or at least without a microscope. Woodland Strawberry is chromosomally diploid, with 2 sets of 7 chromosomes, or 14 total. But Wild Strawberry is octoploid, with 8 sets of 7 chromosomes for 56 total. Polyploidy* is rampant in strawberries with different species possessing 4, 5** 6, 8 and 9 sets of chromosomes***. In the last decade, an Asian species, F. iturupensis, of the Kurile Islands (Russia, Pacific coast) has been discovered to sometimes possess 10 sets- or 70- chromosomes!

*I initially explained polyploidy in this post. But I’ve covered so many examples of it since then, if you don’t know about it by now you are just not paying attention…

**We’ll see a pentaploid strawberry later in the post.

***Triploids don’t occur naturally, but have been produced as cultivated hybrids.

There’s some suggestion that polyploidy may (but not always) be associated with larger fruits. The strawberries we buy at the supermarket- and to which we will return later in the post- are octoploid. The Woodland Strawberry genome, BTW, has recently been fully sequenced. Its small size* and short generation time of 14-15 weeks (in a greenhouse) make it an excellent model genome for Rosaceae, the Rose family, with whose many other members most of its gene sequences are shared.

*The genome, not the plant. Although the plant is small too, I suppose. But that’s beside the point.

Despite having a “typical” genome*, the “fruit” of a strawberry is anything but typical. In fact, botanically speaking it’s not technically a fruit. Actually, even that’s not quite right: the part you think of as the fruit isn’t, but there is real fruit on each strawberry- dozens of them in fact.

They’re the little “seeds” sprinkled all over the skin of the “berry”, which aren’t actually seeds- it’s like everything is bass-ackwards with this “fruit”- but achenes, the same dry fruit architecture we’ve looked at before in everything from Sunflower “seeds” to Dandelion “seeds.” Fragaria achenes are thin-walled, each containing a single seed.

*At least in the case of diploid F. vesca.

Botanically speaking, a fruit is a seed ovary. The red juicy part of the “berry” isn’t ovary; the ovaries of the little white flower will eventually develop into the achenes on the outside of the red juicy part. The red (well, not always, many strawberries are more white in color) juicy part is technically a pseudocarp, the botanical term for a fruitlike structure composed of tissue that is not derived from the ovary wall. Pseudocarp tissues come from other parts of the flower, such as the calyx, or- as is in the case in a strawberry- the receptacle, which is the end of the stem to which the various parts of the flower are attached. In a strawberry, following fertilization, the receptacle develops and swells to many times its size, creating the “berry” we eat.

Lots of other fruits we eat are pseudocarps. Figs- which we looked at last year down in Costa Rica- are formed out of a big, hollow receptacle, with the flowers- which are fertilized by symbiotic Fig Wasps- attached to the inside wall. A pineapple is a pseudocarp that’s sort of a receptacle-conglomerate- grown from the receptacles (and other parts) of many separate flowers. Pome fruits, such as apples and pears, are also pseudocarps; the core is the actual fruit. A pseudocarp is also called an accessory fruit*.

*Older sources also sometimes call them “false fruit”, but I guess that term’s not really used by botanists anymore. Maybe they were afraid of hurting the fruit’s feelings or something.

Strawberries are a very effective dispersal strategy. Many birds eat them, and pass (undigested) the achenes, far from the parent plant. A great example is the Chilean Strawberry, F. chiloensis, whose range spans the West coast of the Americas from Chile to Alaska, its seeds having long ago been spread by migrating birds.

But one of the most interesting and unusual things about strawberries are their relationship with humans, and the history of their domestication and cultivation.

It’s interesting to think about where the fruits and vegetables we eat were originally domesticated. Wheat, barley, rice and millet are all native to, and were domesticated in, the Old World. Corn, potatoes and squash were first domesticated in the New World. Some crops have a back-and-forth history: Sunflowers (which we looked at in this post) are native to the New World, but were artificially selected/domesticated in the Old World.

But strawberries are different* in that they were domesticated independently in the Old and New Worlds. To be fair, the species domesticated were different. F. vesca was first domesticated in Ancient Persia, and it is still cultivated and used in Europe for various sauces and jams. F. chiloensis was domesticated in South America, and bred for larger size.

*Unique maybe? Is there any other crop domesticated in both Old and New World? I don’t know.

But neither the Woodland nor the Chilean Strawberry is what we buy in the supermarket. Commercial strawberries are F. x ananassa, a hybrid of 2 strawberries we’ve already mentioned- the Chilean and Wild Strawberries. Both are New World species, but in another back-and-forth story, their hybridization and domestication happened in Europe, and even more interestingly, by accident, F. virginiana and F. chiloensis being grown in the same garden hybridized in the 19th century. The hybrids display both the sweet flavor of virginiana and the size of chiloensis.

F. x ananassa is chromosomally octoploid, like both of its parents. Both Wild and Chilean parents provide 4 sets of, or 28, chromosomes. But in California, Chilean and Woodland strawberries sometimes hybridize naturally. When they do, the Chilean (octoploid) parent provides 4 sets of, or 28, chromosomes, and the Woodland (diploid) parent provides 1 set, or 7, chromosomes resulting in the naturally-occurring pentaploids I alluded to above. It’s not known whether these 35-chromosome hybrids, known botanically as F. x bringhurstii, can reproduce sexually, but since strawberries also reproduce vegetatively via runners, they’re not uncommon.

Next time you eat a strawberry, look at it and think about it for a moment. Imagine if the receptacle didn’t swell up and get red and juicy, but just stayed, well a receptacle. And instead the fertilized ovules, instead of becoming tiny, hard, dry achenes, swelled up, each into a juicy little fruit of their own. You’d have something completely different. You’d have (more or less) a Raspberry*.

*Or a Blackberry, or- for a local native example we’ve looked at previously- a Thimbleberry, or pretty much any “berry” in the genus the Rubus. I just picked Raspberries because they’re red. And delicious. And the seeds always bother me in Blackberries. Technically, BTW, Raspberries/Blackberries/Thimbleberries et al aren’t true fruits either, but rather aggregations of multiple separate fruits. Blackberries, I’ve heard, have a frighteningly complicated/confusing taxonomy. Let’s hope I don’t run across any too soon.

It’s cool to think about “what-ifs” in evolution and nature. It’s even cooler when it turns out that a “what-if” is real after all. In the case of Strawberries and Raspberries, you can check out a real-life what-if pretty much anytime you want at the local supermarket.

10 comments:

Very thorough. There's a similar story with rice. Philippines IRRI IR series of rice blends quite complex, and alonside of this the masses of varieties of wild rices. Much more to strawberries and rice and a lot else of what we take for granted I guess.

The gourd, Lagenaria siceraria, is a very likely contender. Indications of its use on both sides of the Atlantic are probably too early to be indicative of human dispersal.

Blackberries, I’ve heard, have a frighteningly complicated/confusing taxonomy. Let’s hope I don’t run across any too soon.

Fortunately, not likely to be a problem. Because blackberries still form a coherent group, you can do what most other botanists seem to do and just pretend that all that complicated taxonomy doesn't exist (it's the "Rubus fruticosus aggregate").

One thing I've appreciated from your blog posts is how often things are not what they seem. Pedals that are really sepals, plants that look similar are really very distant relatives, plants with bilogy much more simple than ours have way more chromosomes than we do, and now fruit that's not the real fruit.

Even on the surface life appears complex, but the full story is outright bizarre.

If you buy strawberry plants at the nursery, there are lots of options, but none ever have the scientific name. What I'd really like is a scale of how good they'll taste. Anyhow, that's all beside the point, isn't it?

I looked into berries on wikipedia (because my research skills are very limited) a while ago, and found the whole berry/aggregate fruit/false fruit world confusing and slightly shocking. Cherries are berries, right? I think that's what I read.

And vegetables/fruits aren't distinct in the scientific world, so saying a tomato is actually a fruit seems silly. Saying it's a berry, though, is okay. If it's true.

Botanically speaking, 'fruit' has a specific definition but 'vegetable' doesn't.

Cherries are drupes. Drupes are fruits like plums and apricots, fleshy with a single stone in the middle. Berries usually have multiple seeds and the seeds don't have a hard external case like a stone.

Ah, the gourd. I just knew there had to be something. Thanks Christopher, and thanks for answering Rachel's cherry/berry/drupe question.

Rachel- As to how they taste, I haven't tried any "wild" strawberries, though supposedly they really do have different flavors. Woodland Strawberries are often called "Alpine Strawberries" at nurseries, and supposedly are delicious (though tiny.) I guess strawberries are similar to bananas, in that there are lots of different varieties, each with their own texture and flavor, but the vast majority of which can't be shipped to a supermarket intact and then taken home right around ripening time.

You'd come across blackberries on a regular basis if you lived in Boise. There's a bush that conveniently grows next to a gate that you have to stop and open along a popular trail in Boise. Since you're stopped and off the bike to pass through the gate, you may as well grab a handful of blackberries to snack on before you start riding again.

The first crop that came to my mind as having been domesticated in both the old and new world is cotton. This is unequivocal as the new world ones are polyploid and the old world ones diploid. (Does this sound familiar?)

On the other hand, there is some recent evidence that Lagenaria might actually have been brought to the Americas already domesticated, despite its early dates. See Erickson, & al. 2005 in PNAS.

Fragaria vesca is very common here (UK), in hedgerows. Although the fruits are small, when fully ripe they pack quite a flavour hit, somewhat reminiscent of bubblegum - the esters I guess. If the weather allows (like this year), they dry down nicely on the plant and can be chucked into breakfast cereal like raisins. The alpine strawberry a non-running form of F. vesca, without stolons is quite commonly grown in gardens. Tastes just as good.